The past several years have seen a paradigm shift in prostate cancer (PCa) therapy; and it is now becoming widely accepted that androgen receptor (AR) remains active and is a therapeutic target in prostate cancers that relapse after surgical or medical castration (castration resistant prostate cancer, CRPC).'This Program Project brings together a group of investigators with extensive and complimentary expertise in androgens and AR in PCa, and with a track record of accomplishments and productive collaborations. Each project focuses on distinct mechanisms that contribute to AR activity and function in CRPC. Project 1, Steroid Metabolism in Castration-Resistant Prostate Cancer (PI Peter Nelson), focuses on targeting intratumoral androgen synthesis and mechanisms of resistance to abiraterone and other inhibitors of androgen synthesis. Project 2, Basis for Androgen Receptor Antagonist Resistance in CRPC (PI Steven Balk), focuses on mechanisms of action and resistance to AR antagonists. Project 3, Development of Castration Resistance by Alternative AR Splicing (PJ Stephen Plymate) focuses on the role of alternative AR splicing in CRPC. Project 4, Epigenetic Reprogramming of AR Function In CRPC (PI Myles Brown) focuses on the AR transcriptional program and how it is altered with PCa progression. Core A, Administrative/Clinical/Biostatistics Core (PI Steven Balk, Co-PI Peter Nelson) will coordinate the overall program, provide biostatistical support, facilitate access to patient materials, and consult on approaches to enhance/accelerate translation to the clinic. Core B, Biospecimen and Animal Models Core (PI Robert Vessella), will provide a unique series of PCa xenograft models in conjunction with the expertise and infrastructure to carry out trials of single and combination therapies in these models. Dr. Vessella also directs a very robust biospecimen collection and processing Core, and will provide further access to appropriate clinical materials. Core C, Steroid Analytical Core (PI Trevor Penning), will develop and deploy needed state-of-the-art methods to measure multiple steroid and metabolites in human and mouse samples.